1. Field of the Invention
The present invention relates generally to a shutter for use in a camera and, more particularly, to a camera shutter having exposure control means that performs an opening operation by an opening sector and a closing operation by a closing sector.
2. Description of the Related Art
A shutter that controls exposure by an opening sector and a closing sector has opening and closing levers for performing opening and closing operations, respectively. The opening and closing levers are spring-biased in opening and closing directions and are held in their charged condition by electromagnets. The electromagnets are deenergized whereby the opening and closing levers are driven by the stored spring forces to perform opening and closing operations. Exposure is controlled by varying the timing at which the opening and closing electromagnets are deenergized. A synchronous contact member for producing a trigger signal for a strobe is mounted near the position at which the operation of the opening lever ends. The opening lever fully opens the opening sector and simultaneously therewith, the synchronous contact member is energized to produce the trigger signal to cause the strobe to flash.
One known structure is shown in FIG. 6, where a shutter base plate 1 is provided with a shutter opening 1c. An opening lever 21 is rotatably fitted over a shaft 1a standing on the shutter base plate 1. The opening lever 21 is biased in a counterclockwise direction by an opening spring (not shown). Similarly, a closing lever 22 is rotatably fitted over a shaft 1b on the shutter base plate 1, and the closing lever 22 is biased in a counterclockwise direction by a closing spring (not shown). The opening lever 21 and the closing lever 22 have protrusions 21b and 22b at their respective front ends. The protrusions 21b and 22b pass through arc-shaped holes 1e and 1f, respectively, formed in the shutter base plate 1, and the protrusions protrude below the shutter base plate 1. An opening sector and a closing sector (neither of which is shown) are driven by the opening lever 21 and the closing lever 22, respectively, to open and close the shutter opening 1c. 
A charge lever 30 is rotatably fitted over a shaft 1d on the shutter base plate 1. The opening lever 21 and the closing lever 22 are placed into a charged position by the charge lever 30. The motion of the opening lever 21 and the closing lever 22 is controlled by an opening electromagnet 41 and a closing electromagnet 42 under control of a CPU (not shown) incorporated in the camera. A printed circuit board 60 is supported on the top ends of the shafts 1a and 1b standing on the shutter base plate 1. The opening electromagnet 41 and the closing electromagnet 42 are soldered to circuits on the printed circuit board 60 and electrically connected with these circuits. opening contact members 151a, 151b and closing contact members 152a, 152b are crimped to the printed circuit board 60.
FIG. 5 shows portions for activating and deactivating the opening contact members 151a and 151b of a shutter similar to the shutter of FIG. 6. Note that elements or components performing like functions are indicated by like reference numerals in both FIGS. 5 and 6. Each of the opening contact members 151a and 151b is fabricated by bending a metal leaf spring in a complex manner to crease the spring. In the state of FIG. 5, the opening sector has been displaced into the charged position, thus closing the shutter opening 1c. Under this condition, the opening sector is attracted and held by the opening electromagnet 41. At this time, the opening contact members 151a and 151b are opened. When the shutter opening 1c is opened, the opening lever 21 is released from the opening electromagnet 41 and rotated by the opening spring in a counterclockwise direction. A pin 21a mounted on the opening lever 21 is pushed against the opening contact member 151a near the position at which the counterclockwise travel of the opening lever 21 ends. The opening contact member 151a is thereby pushed against the opening contact member 151b, closing both contact members. Thus, both contact members are electrically connected with each other. In this structure, if the shutter opening 1c is not fully opened by the opening sector, an opening signal indicating opening of the contact members is produced. If the shutter opening is fully opened by the opening sector, a closure signal is produced. Since the closing contact members 152a and 152b are substantially identical in structure with the opening contact members 151a and 151b, a description thereof has been omitted.
In this way, in the prior art structure, when the shutter opening 1c is opened by the opening sector, the opening contact members 151a and 151b are closed and electrically connected together. When the shutter opening 1c is closed by the closing sector, the closing contact members 152a and 152b are closed and electrically connected. Therefore, immediately after the contact members are closed, chattering tends to occur because of the resilience of the contact members. In the case where the opening contact members 151a and 151b are fabricated as synchronous contact members and an electric signal is used to trigger flash of the strobe, the chattering may produce a false trigger signal for flash of the strobe. As a result, the strobe may erroneously flash. Furthermore, each of the contact members 151a, 151b, 152a, and 152b is fabricated by bending a metal leaf spring in a complex manner to crease the spring, which increases the fabrication cost. Another problem is that there are a large number of input and output terminals between the CPU in the camera and the driver circuit of the shutter, which complicates assembly.
Accordingly, it is an object of the present invention to provide a focal plane shutter which uses a synchronous contact member thereby resulting in a decreased amount of chattering.
Another object of the present invention is to provide a focal plane shutter having a synchronous contact member which is easy and inexpensive to fabricate and assemble in the shutter.
A further object of the present invention is to provide a focal plane shutter having a synchronous contact member and which has a reduced number of input and output terminals.
Another object of the present invention is to provide a focal plane shutter having a synchronous contact member which produces an output signal used as a trigger signal for flashing a strobe thereby securing generality of use and design.
A focal plane shutter in accordance with the present invention has a shutter base plate provided with a shutter opening, an opening sector for opening the shutter opening, a closing sector for closing the shutter opening, and a normally closed synchronous contact member linking the opening sector and an output circuit. When the shutter opening is opened, the synchronous contact member is opened. The output circuit produces an opening signal when the synchronous contact member is closed. When the synchronous contact member is opened, the output circuit produces a given output signal different from the opening signal. Since the synchronous contact member, which is normally closed, is opened when the shutter opening is opened, the amount of chattering is reduced. The shutter can be assembled and inspected in a shorter time. Misflash of the strobe can be prevented. Furthermore, because an opening signal is produced when the synchronous contact member is closed, and because a given signal different from the opening signal is produced when the synchronous contact member is opened, a signal corresponding to the degree to which the shutter opening is opened by the opening sector can be obtained thereby securing generality of design.
In accordance with the invention, the focal plane shutter includes a closing electromagnet for controlling the time when the closing sector starts to operate. The output circuit includes a transistor and a first resistor. A driver voltage for the closing electromagnet is applied to the base of the transistor via the first resistor. The collector of the transistor is connected with a higher-potential side of a power supply via a second resistor. The emitter of the transistor is connected with a lower-potential side of the power supply. First and second voltage signals are produced from the collector of the transistor. One end of the synchronous contact member is connected to the lower-potential side of the power supply, the other end being connected to the base of the transistor. In addition, energy saving can be accomplished, because an electric current flows through only the synchronous contact member when a driver voltage is applied to the closing electromagnet. Additionally, the closing electromagnet and the transistor can be controlled with the same voltage signal, because the driver voltage for the closing electromagnet is applied to the base of the transistor. Terminals can be shared between the focal plane shutter and the CPU in the camera, thereby reducing the number of terminals which would otherwise be needed.
The synchronous contact member is comprised of a linear spring and a pin. Therefore, it is easy to fabricate the synchronous contact member at low cost.